Except that government has a lot more political power than your average philanthropist. It's not even close. Tax money inherently involves political power, in a way that's simply not the case for philanthropic donations.
Strictly speaking, the U.S. is a republic not a democracy. Are you sure that your average congresscritter makes better decisions on money allocation at the margin than a private philanthropist would?
Microsoft and Amazon don't care what color my house is or how close my garage is to the property line so long as I keep paying them. Just paying my property taxes is not enough to prevent arbitrary violation of my property rights by my local government.